1. Field of the Invention
The present invention relates to a radiation monitor and particularly to a radiation monitor which confirms soundness of a steam generator in a pressurized water reactor plant.
2. Description of the Related Art
There is a radiation monitor which is called a sensitive main steam pipe monitor among radiation monitors which confirm soundness of a steam generator by monitoring leakage from a primary coolant to a secondary coolant of a steam generator (SG) of a nuclear power plant. This sensitive main steam pipe monitor includes: a radiation detector which is disposed close to a main steam pipe and detects a radiation to output an analog voltage pulse; and a count-rate measurement unit which receives the analog voltage pulse, discriminates the analog voltage pulse entering a high-energy window which is set to contain a photoelectric peak, a single-escape peak, and a double-escape peak of γ-ray (6.13 MeV) of N-16 which is a radionuclide contained in the steam in the main steam pipe, to output a digital pulse, and measures a count rate of the digital pulse, and monitors a change in the count rate.
In the same manner as a typical count-rate measurement unit of a radiation monitor, the count-rate measurement unit of the sensitive main steam pipe monitor counts digital pulses which are discriminated by pulse heights, and acquires and outputs a count rate by performing a time constant process using software so that a standard deviation becomes constant based on the counted value. It is also possible to have a suitable response according to the purpose, by switching the standard deviation according to the count rate. If necessary, the plurality of count rates can be acquired by performing the plurality of time constant processes, and the plurality of count rates having different standard deviations can be displayed for comparison (for example, see PTL 1).
In addition, a technology of determining whether an indication increase occurs by synchronizing the upper stream and the lower stream of the main steam pipe in two detecting positions, depending on a signal or noise has also been proposed (for example, see PTL 2).
The steam in the main steam pipe is in a secondary system and does not contain artificial radionuclides in a normal state. In addition, a background count rate in a normal state is low as approximately several cpm because cosmic radiations are dominant, and the background count rate and an alert setting point are close to each other. Accordingly, when alert transmission is attempted at high precision by preventing erroneous alerts, the standard deviation is reduced, and as a result, the response of the alert transmission is delayed, and when the standard deviation is increased by giving priority to the response of the alert transmission, erroneous alerts may frequently occur. Therefore, the alert is divided into two stages which are a caution alert and a high-level alert which is at an upper level of the caution level. The caution alert is transmitted during a stage of a slight leakage, and investigation is minutely performed by including a possibility of the erroneous alerts.
[PTL 1] JP-A-61-128184 {Expression (1), Expression (4), FIGS. 1, 5, 6, and 10 to 16}
[PTL 2] JP-A-4-268496 (FIGS. 1 and 2)
The radiation monitor of the related art is configured as described above. Since the analog voltage pulse from the radiation detector is input to the count-rate measurement unit, the pulse height values entering the set window are discriminated and counted, the time constant process is performed us ing software so that the standard deviation becomes constant based on the counted value, the count rate is acquired and output by giving priority to the responsiveness, and the alert setting point is close to the background count rate, the alert may be erroneously transmitted due to a statistical change, so-called fluctuation, of the count rate, and it is necessary to perform an operation of performing off-line inspection of an apparatus, to be safe, to confirm soundness, even when the count rate is restored to the background count rate.
With respect to this, in a method of acquiring the plurality of count rates having different standard deviations from the same input, for comparing the change thereof, approximately 20 minutes are taken for the regular output of the count rate obtained by giving priority to the responsiveness to approach an apex of the fluctuation, and when this changes in a state where the count rate for diagnosis having low responsiveness follows, it is difficult to identify the reason because the input is in the same pulse stream.
In addition, as disclosed in PTL 2, in the proposal of comparing changes of count rates of two detecting positions of the upper stream and the lower stream of the main steam pipe, the background count rate is small as a several cpm, and accordingly, a possibility that increasing tendencies become the same, cannot be ignored, and there is no fundamental resolution disclosed.